Specifically, the way I would do it is link an LFO to the OSC pitch of some moddepth (say, 2 semitones). Then, assuming your LFO has a setting for its amplitude, I would link the Modwheel (of some [moddepth]%) to that amplitude (adjust either [Modwheel moddepth]% or [LFO amplitude]% to match the vibrato intensity you want). As a result, you can move your Modwheel to modulate between no vibrato and [Modwheel moddepth/100 x LFO Amplitude/100]% vibrato.
The reasoning for this involves the idea that you'd be using modwheel to modulate LFO amplitude, which controls the modulation depth of the LFO, which (the LFO) controls the pitch. Why such a chain? This way, you have control over when the vibrato comes in (modwheel), at what intensity (LFO amplitude), and over what pitch range (LFO moddepth) it can reach.
At this point, after that setup, you would have the Pitchwheel for the Master pitch, and the Modwheel for the vibrato mix level (note that if the Modwheel has a moddepth of X%, it means X% of the LFO amplitude, so if the Modwheel is modulating 100% of the LFO amplitude and the LFO amplitude itself is at 60%, the Modwheel makes the synth use 60% "vibrato" and 40% "no vibrato").
If you have any questions, please try to post a screenshot of your synth, and then we can see how to do it specifically.